Simulations reveal previously unknown 2D phthalocyanine monolayers combining structural stability, high mobility, and direct bandgaps.
(Nanowerk Spotlight) Two-dimensional materials have rapidly expanded from a niche curiosity to a core area of materials research. Graphene’s discovery catalyzed exploration into atomically thin systems, inspiring efforts to discover, synthesize, and characterize materials with diverse electronic, optical, and mechanical properties.
From transition metal dichalcogenides to boron nitride and beyond, these ultrathin systems have found relevance in fields ranging from photonics to quantum computing. Yet despite this momentum, the space of 2D organic or organometallic systems remains underexplored. One of the most promising, yet experimentally elusive, candidates in this area are phthalocyanine-based monolayers.
Phthalocyanines—macrocyclic compounds structurally related to porphyrins—have been used extensively in molecular electronics, sensors, photovoltaics, and organic semiconductors. Their central cavities can coordinate various metal ions, granting tunability of their optical and electronic behavior. While transition metal-based phthalocyanines have been synthesized and studied for decades in molecular form, only relatively recently have their two-dimensional monolayer counterparts, such as FePc, been prepared and characterized.
These transition metal phthalocyanine (TMPc) monolayers opened up possibilities for combining molecular-level tunability with long-range 2D order. However, the potential of phthalocyanine monolayers using main group elements from the p-block—a class with strong optical activity and low toxicity—has remained largely theoretical. These main group phthalocyanines (mgPc) are abundant in molecular form but have not yet been realized as 2D materials, leaving a major gap in the design landscape for functional monolayers.
This landscape is now reshaped by a study published in Advanced Energy Materials (“2D p‐Block Main Group Phthalocyanine Monolayers”). Using high-level quantum mechanical simulations, researchers from Huazhong University of Science and Technology and the University of California, Berkeley have systematically characterized an extensive library of 2D mgPc monolayers.
Their work reveals 21 previously unreported semiconducting monolayers, many of which show electronic transport properties rivaling or exceeding those of known 2D materials. By varying the central main group atom, attaching axial ligands, and applying biaxial strain, the authors demonstrate fine control over conductivity, bandgap, and carrier mobility—three central parameters for electronic and optoelectronic design. The study also uncovers eight materials hosting Dirac cones, a hallmark of massless charge carriers, and predicts high-efficiency solar cell heterostructures made entirely from mgPc monolayers.
Demonstration of different types of single-layer structures through a diagram. This is the skeleton of mgPc monolayer that displays different types of unit cells representing different types of single-layer structures (including flat, wrinkled, mono-substituted, di-substituted, etc) labeled with different letters. a) shows 12 main group central elements of mgPc monolayers. b–d) are the tilt b) and side c,d) views of the unit cell of mgPc monolayers, respectively. e) and f,g) are the tilt views of mgRPc and mgR2Pc monolayers, respectively. The light blue, brown, peach, and cyan balls represent N, C, H, and mg atoms, respectively. (Image: Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
The researchers began by computationally constructing phthalocyanine monolayers with central atoms drawn from groups 13 through 16 of the periodic table. These elements include Al, Si, P, S, Ga, Ge, As, Se, In, Sn, Sb, and Te. Using spin-polarized density functional theory and molecular dynamics simulations, the team examined how each element interacts with the surrounding phthalocyanine scaffold. They extended their analysis by attaching one or two axial ligands—atoms or groups like chlorine, hydroxyl, or oxygen—above and below the central atom.
These variations yielded more than 60 unique systems including bare mgPc, mono-ligated mgRPc, and di-ligated mgR2Pc structures. The team calculated equilibrium structures, electronic band structures, mechanical stability, optical absorption spectra, and thermodynamic stability across all configurations.
A primary finding of the study is the discovery of 21 monolayers that behave as semiconductors. Most retain direct bandgaps even under strain, a critical feature for efficient light absorption and emission. Importantly, the bandgap values of these semiconductors are tunable via strain engineering, ranging from 0 to 1.59 eV as calculated using a hybrid exchange-correlation functional (HSE06). This wide bandgap coverage enables adaptation of mgPc materials for diverse applications.
The evolution of the bandgap under strain follows a “unimodal model”—an initial increase in gap with tensile strain followed by a decrease past a maximum, and a mirrored narrowing under compressive strain. In eight cases, compressive strain closes the gap entirely, giving rise to Dirac-like band crossings at the Fermi level.
These Dirac cones appear at high symmetry points in the Brillouin zone and exhibit distinct geometries. Some are symmetric around the Fermi level, while others show asymmetry with one side flattened. These cones reflect linear dispersion, suggesting charge carriers behave as if massless near these points. The associated Fermi velocities reach values above 6 × 10⁵ meters per second in some materials—approaching those found in graphene and exceeding those of silicene, germanene, and stanene. These features make mgPc monolayers promising candidates for high-speed electronics and low-power devices.
In addition to Dirac behavior, the calculated carrier mobility of these semiconductors is unusually high. Five of the materials exhibit mobilities exceeding one million square centimeters per volt-second—a range rarely achieved even in the most optimized 2D systems. Thirteen surpass 10⁵ cm²/V·s. These values were obtained using deformation potential theory, which takes into account effective mass, phonon scattering, and elastic moduli. Interestingly, hole mobility is generally higher than electron mobility in these systems, with GeCl₂Pc, SnCl₂Pc, and SiCl₂Pc reaching hole mobilities above one million.
Axial ligands are shown to enhance mobility in several cases by modifying local bonding environments and electronic distributions. For example, adding chlorine to SPc boosts hole mobility from 1.07 × 10⁵ to 4.34 × 10⁵ cm²/V·s.
Calculated carrier mobility (μ) of the mgPc, mgRPc and mgR2Pc monolayers. (Image: Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
The study also evaluates optical absorption, a key requirement for solar energy applications. The calculated absorption coefficients of selected mgPc monolayers exceed those of many typical photovoltaic materials and show high intensity in both visible and ultraviolet ranges. Materials such as AlClPc and SiCl₂Pc absorb strongly across a wide energy window, with absorption coefficients over 7 × 10⁵ cm⁻¹ in the UV range. This broad and intense optical activity, combined with direct bandgaps, supports their suitability as light harvesters.
To assess their performance in solar cells, the authors constructed theoretical heterojunctions between pairs of mgPc semiconductors. Using Scharber’s method, they estimated the power conversion efficiencies (PCEs) for 134 type-II band-aligned heterojunctions. These configurations are favorable for efficient charge separation. Among these, several combinations exceeded 24% PCE, with a maximum of 26.28% for a heterojunction composed of PClPc and P(OH)Pc. This figure exceeds many proposed 2D photovoltaic architectures and suggests that all-mgPc heterostructures could be viable candidates for flexible solar technologies.
Mechanical and thermal stability are essential for real-world integration. All studied mgPc monolayers show mechanical robustness, with positive elastic constants satisfying Born criteria. Simulated annealing at 500 K confirmed that their structures remain intact under elevated temperatures. Phonon dispersion calculations revealed no imaginary modes, indicating dynamic stability. Together, these results suggest that mgPc monolayers could be experimentally realizable and stable under operational conditions.
At the electronic level, the study offers insight into bonding characteristics using advanced chemical analysis tools. The phthalocyanine scaffold supports aromatic π-electron delocalization across a 16-membered ring, evidenced by multi-center bonding patterns uncovered through adaptive natural density partitioning. This kind of electron delocalization had not been previously reported in 2D periodic systems. The bonding picture varies depending on the oxidation state of the central atom and the nature of attached ligands, influencing electronic structure and charge distribution.
Taken together, this computational survey establishes a new class of 2D organic-inorganic hybrid semiconductors with adjustable properties and strong electronic and optical performance. The tunability across bandgap, mobility, and chemical composition enables precision design of devices tailored to specific needs. Their combination of high carrier mobility, Dirac behavior, and broad light absorption provides multiple entry points for potential application in nanoelectronics, photodetectors, and solar energy conversion.
Although these materials remain theoretical at present, their strong predicted stability and functional diversity make them compelling targets for experimental synthesis. Their modular chemical structure—based on well-known phthalocyanine cores—further raises the possibility of bottom-up assembly via solution-phase or vapor deposition techniques. This work lays the foundation for extending the reach of 2D materials into the realm of tunable organic monolayers built from p-block main group elements.
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